fix(ui): stop wheel scroll and stale hover from drifting comment targets off the highlighted line - #1143

…ets off the highlighted line

A 'c' comment could open its draft a few lines (or one collapsed gap)
away from the highlighted line. Two paths let the line cursor drift away
from the current line:

- Wheeling past the highlighted line clamped the cursor to a viewport
  edge, and wheeling back never restored it; the strand survived the
  whole roundtrip and deepened with distance.
- A hover add-note affordance stayed armed after the cursor stepped
  away, so the next implicit comment targeted the hovered row instead of
  the highlight.

The clamp now remembers the stop it displaced, unwinding one hop per
scroll step back, and a deliberate cursor move ends the unwind; cursor
movement retires a stale hover. Keyboard-only commenting was always
exact and is unchanged.

A vertical drag across exactly two rows was also swallowed: the click
slop that ignores one cell of pointer jitter applied to the row axis, so
a two-row gesture read as a jittered click and painted no selection or
action bar. The slop is now horizontal only.

Co-authored-by: GLM-5.3-Flash